Effect of pea plant architecture on spatiotemporal epidemic development of ascochyta blight (Mycosphaerella pinodes) in the field

The effect of pea canopy structure on epidemics of Mycosphaerella pinodes was investigated in four spring pea cultivars (Bridge, Obelisque, Solara and Athos) in two field experiments. These cultivars had similar levels of susceptibility to ascochyta blight and presented different architectural features (branching, standing ability and stem height). Stem height ranged from 77 to 95 cm in 1999 and from 72 to 92 cm in 2000, while leaf area index (LAI) ranged from 3·8 to 5·1 in 1999 and from 3·8 to 4·7 in 2000. Internode lengths varied from 4·6 to 6·0 in 1999 and from 3·8 to 4·7 in 2000. Mean distance between nodes in the canopy (MDN) ranged from 12·2 to 15·3 cm in 1999 and from 11·2 to 13·9 cm in 2000. Canopies with different architecture differed in disease progression on stipules and mainly on pods. The levels of disease on stipules and pods were strongly correlated. Moreover, if disease was considered as a function of stipule height up the stem, large differences in vertical disease distribution were observed between cultivars. Three architectural features acted on disease development: cumulative LAI above the node on which disease was assessed (LAIcum) and large MDN favouring disease development, and large internode length reducing disease severity. Modifying LAI distribution and plant organization could be one way to reduce the impact of ascochyta blight, by direct or indirect effects on environmental and dispersal conditions.     

Effect of weather conditions on local spread and infection by pea bacterial blight (Pseudomonas syringae pv. pisi)

The effect of weather conditions on simultaneous local (plant to plant) spread and infection of peas (Pisum sativum) with bacterial blight (Pseudomonas syringae pv. pisi) was investigated by exposing susceptible bait plants for 24 h periods in infected field plots. Following exposure, bait plants were maintained in a glasshouse. Disease symptoms were recorded on 55 out of a total of 105 days on which plants were exposed. Nearly all of these infection events (53) were associated with the occurrence of rain. A series of Generalised Linear Models was fitted to the data to examine the relationships of the mean number of lesions (m) or the proportion of bait plants infected (p) to various weather variables and disease levels in the plots. Rainfall rate and wind run were the most important explanatory variables for the mean number of lesions followed by maximum temperature, rainfall duration, rainfall in the previous week and disease incidence in the surrounding crop. However, rainfall duration and disease incidence were the most important for the proportion of bait plants infected, followed by wind run. A four variable model relating the mean number of lesions to the rainfall rate, wind run, maximum temperature and either rainfall the previous week or disease incidence in the surrounding crop was considered to be the most useful for use in simulation studies.     

Effects of temperature and moisture on disease and fruit body development of Mycosphaerella pinodes on pea (Pisum sativum)

The effects of temperature (5–30°C) and the duration of moisture on the development of ascochyta blight ( Mycosphaerella pinodes ) on pea seedlings, grown under controlled conditions, were investigated. The optimum temperature for monocyclic processes was 20°C. At this temperature, pycnidiospores germinated after 2 h, appressoria formed after 6 h and the germ-tube penetrated the leaf cuticle after 8 h. Disease symptoms were evident after 1 day of incubation and the first pycnidia formed after 3 days. Longer wetting periods were required for disease development and pycnidial formation at non-optimal temperatures. Disease severity and the number of pycnidia formed on leaves increased with temperature from 5 to 20°C, then decreased between 20 and 30°C. Polynomial equations were fitted to predict the stages of infection, incubation, latency and disease development as functions of temperature and duration of moisture. These equations allow comparisons of pathogen spread with plant development and could be incorporated into disease development models used for crop management programmes.     

Effects of interrupted wet periods and different temperatures on the development of ascochyta blight caused by Mycosphaerella pinodes on pea (Pisum sativum) seedlings

The effect of interrupted wet periods on pycnidiospores of Mycosphaerella pinodes was studied by assessing spore viability, infection and disease development on pea seedlings. Pycnidiospores survived dry periods of up to 21 days after inoculation. Rewetting restored the infection capacity of the pycnidiospore, resulting in high levels of disease. The effects of wet–dry–wet cycles depended on when the dry period occurred during the infection process. No disease symptoms appeared when dry periods occurred during germination. A low level of disease occurred after rewetting in high relative humidity if the interruption of the wet period was long. However, a wet period resulting in leaf wetness after a dry period gave similar levels of infection to those achieved with a continuous wet period. Pycnidiospores formed appressoria but hyphae did not penetrate if a 6–12 h wet period preceded the dry period, and only a few flecks appeared during the dry period. Coalescent necrosis occurred when the dry period followed penetration. The disease was severe in each case when plants were returned to wet conditions after a period of dryness. Lesion development depended on the duration of the initial wet period, and the characteristics (temperature and duration) of both the dry period and the final wet period.     

Effect of soil moisture on the transmission of pea bacterial blight (Pseudomonas syringae pv. pisi) from seed to seedling

Controlled-environment studies in which pea seed cv. Solara, inoculated with Pseudomonas syringae pv. pisi , was sown in pots of compost maintained at different moisture contents showed that soil moisture had a considerable influence on the transmission of the disease from seed to seedling. An equation was derived from this data which described the relationship between the proportion of seedlings infected ( p ) and the soil water stress ( s ) in MPa: -In(-In(1 − p )) = 0.64 In( s ) + 4.5. This equation was used to produce predicted transmission rates for each year from 1987 to 1990, which were compared with measured transmission rates in field experiments at Wellesbourne in the same years. Although agreement between the observed and predicted transmission rates was poor, years of severe and slight disease transmission were successfully predicted.     

Improvement and validation of a pea crop growth model to simulate the growth of cultivars infected with Ascochyta blight (<Emphasis Type="Italic">Mycosphaerella pinodes</Emphasis>)

A model simulating the growth of a pea crop infected with Ascochyta blight was improved and validated using 6 spring pea cultivars, all equally susceptible to Ascochyta blight, but differing in architectural features (stem height, branching ability, standing ability). This model takes into account the spatial distribution of the disease, including the contribution of each layer of the canopy to the radiation interception efficiency (RIE) and the radiation use efficiency (RUE) of the crop. The decreasing contribution of each layer due to the disease was estimated by the relationship between the photosynthesis of a layer and its disease score. The effect of disease on photosynthesis was assessed in controlled conditions as a means of evaluating the effect of disease on each cultivar. All cultivars were affected equally. In field conditions, cultivars with different canopy architectures displayed differences in the profile of disease on leaves. Cultivar Aladin reached higher disease levels at the top of the plant. Epidemics affected crop growth, and the cultivars tested differed in the magnitude of the decrease in growth. Observed and simulated data were compared. The disease-coupled crop growth model gave satisfactory predictions of crop growth for the six cultivars tested.